1. Field of the Invention
The present invention relates in general to air conditioning systems, and relates in particular to so-called hybrid air conditioning systems that combine desiccant air conditioning and a heat pump device.
2. Description of the Related Art
FIG. 18 is a schematic representation of a conventional air conditioning system combining a ventilation unit 1A for introducing outdoor air for process air (for conditioning space) while discharging indoor air to the outside and an air conditioning unit 3 for circulating process air. The ventilation unit 1A is a total enthalpy heat exchanger and performs heat exchange simultaneously of both latent heat for humidity as well as sensible heat between the indoor air and the outdoor air. In the meantime, air conditioning load within the conditioning space is withdrawn by the air conditioning unit (using a heat pump) 3 and is discarded to the outside environment.
The efficiency of such an enthalpy heat exchanger is low and ranges between 50.about.55%, resulting in that between 45.about.50% of the moisture contained in outdoor air is brought into the conditioning space. This moisture must be removed by the air conditioning unit 3, and therefore, it is necessary for the air conditioning unit 3 to lower the operating temperature of a heat exchanger 4 (low temperature heat source) to a temperature below the dewpoint (15.about.16.degree. C.) of the indoor air, for example to 10.degree. C. The result is that the temperature difference (temperature lift) between the evaporating temperature and the condensing temperature for the air conditioning unit 3 has to be set to a same value as when the enthalpy heat exchanger 1A is not being used, leading to high energy consumption.
Also, the air conditioning unit 3 must have a drain provision to drain the moisture condensation, thus necessitating an additional mechanical arrangement.
Furthermore, because the air conditioning unit 3 had to simultaneously process both the latent heat (moisture removal) and the sensible heat (cooling of process air), the humidity of the conditioning space was affected by the temperature conditioning process, and could not be controlled sufficiently.
There are a few air conditioners having a humidity reduction mode of operation, however, dehumidification efficiency is not sufficient in these units because the basic approach of removing the moisture by cooling leads to a low temperature of the conditioned space.